I’m looking for a magnetic or suction tank bag. I think magnets are the way to go.
ebay.com
newenough.com has a few good bags and there is a really nice one in the clearance section.
I love RKA tankbags. They’re a small company, but they make excellent motorcycle luggage and have great customer service. They replaced a lost rain cover for free, and I’ve had my bag for nearly twelve years of constant use.